The present invention relates to an isolation-type voltage measuring apparatus for measuring a voltage from an electrical apparatus under conditions electrically isolated from the electrical apparatus to be measured, and more particularly to a voltage measuring apparatus with flying capacitor method having a capacitor and a switch.
In recent years, in an electrical system for making voltage measurements, it has been known to use an isolation-type voltage measuring apparatus wherein the electrical apparatus to be measured, for example, a voltage source, is isolated electrically or separated in terms of electric potential from the voltage measuring part. More specifically, in a power supply system for an electric vehicle or a power storage system for a domestic use, a monitor apparatus for monitoring voltage is connected to battery cells with the above-mentioned voltage measuring apparatus, and the voltage from the battery cells is measured and monitored under conditions electrically isolated from the battery cells.
Stated more specifically a high output power source of hundreds of volts for the electric vehicle or the like is configured, as is well known, with connecting in series a large number of secondary battery cells exemplified by stacked- or aggregate-type nickel hydrogen storage cells. To control the charging and discharging of these series connected cells, the performance of each individual cell must be monitored. Further, in the electric vehicle, the high voltage system including the high output power source is electrically isolated from the chassis to prevent hazards. On the other hand, since the processor for controlling the charging and discharging uses the chassis as a reference potential, the cell voltage must be measured in an electrically isolated manner.
A first voltage measuring apparatus for making such voltage measurements, has a monitoring apparatus for a battery pack as disclosed in Japanese Unexamined Patent Publication No. 8-140204. This first conventional voltage measuring apparatus comprises a voltage monitoring unit which includes an operational amplifier, an A/D converter, a photocoupler, and a reference power supply, and is designed to measure the voltage of a 240-cell series battery (stacked-type voltage source) delivering a total voltage of 288 V. More specifically, in this first conventional voltage measuring apparatus, since it is actually difficult to measure and monitor many voltages of large number of individual cell, 10 cells are grouped into one module, and the voltage of each module, in an example, voltages of 24 modules, are measured using a voltage measuring apparatus.
However, in the above-mentioned first conventional voltage measuring apparatus, there was a problem that the configuration of the measuring apparatus is rather complicated because the voltage monitoring unit having a complex construction has to be provided for each module.
A second conventional voltage measuring apparatus, for example, U.S. Pat. No. 5,163,754 discloses a temperature measuring apparatus. This second conventional voltage measuring apparatus measures the output voltage of a thermocouple in an electrically isolated manner by using the earlier described flying capacitor method.
A major section of the second conventional voltage measuring apparatus will be described in detail below with reference to FIG. 23.
FIG. 23 is a circuit diagram showing a major section of a second conventional voltage measuring apparatus.
As shown in FIG. 23, in the second conventional voltage measuring apparatus, a voltage source 101 to be measured is connected to a capacitor 103 via a first switching device 102, and the capacitor 103 is connected to a buffer circuit 105 via a second switching device 104. The first switching device 102 is configured with two switches 102a and 102b which operate in interlocking fashion with each other. Likewise, the second switching device 104 is configured with two switches 104a and 104b which operate in interlocking fashion with each other. Each of the switches 102a, 102b, 104a, and 104b is constructed from an isolated driving type analog switching element having high voltage withstanding capability, for example, a MOSFET with optical driver. The buffer circuit 105 is connected to a known voltmeter (not shown).
In the above-mentioned second conventional voltage measuring apparatus, first, while holding the second switching device 104 in the OFF-state, the first switching device 102 turns on to transfer the voltage of the voltage source 101 to the capacitor 103. Thereby, the capacitor 103 holds the voltage. Next, the first switching device 102 turns off and the second switching device 104 turns on, thereby to input the voltage of the voltage source 101 into the buffer circuit 105. In this way, with the first and second switching devices 102 and 104 not in the ON-state simultaneously, the second conventional voltage measuring apparatus measures the voltage of the voltage source 101 by maintaining electrical isolation from the voltage source 101.
However, in the second conventional voltage measuring apparatus, there was a problem that accuracy of voltage acquisition by and at the capacitor 103 degrades, hence leading to a degradation in voltage measurement accuracy.
The problem of the voltage measurement accuracy degradation in the second conventional voltage measuring apparatus will be explained with reference to FIG. 23.
As shown in FIG. 23, in the case that the voltage source 101 is superimposed on a disturbance voltage (hereinafter also referred to as a xe2x80x9ccommon mode voltagexe2x80x9d) En which is unsteady with respect to ground potential, the instant that the first switching device 102 held in the ON-state turns off and the second switching device 104 turns on, the voltage between both terminals of each of the switches 102a and 102b changes from zero toward the disturbance voltage En. As a result, in the second conventional voltage measuring apparatus, leakage currents Ia and Ib shown in the figure flow based on the change of the charge on capacitances of the respective switches 102a and 102b in the OFF-state. The leakage current Ia passes through the capacitor 103 and flows into ground potential for the buffer circuit 105 together with the leakage current Ib. As a result, in the second conventional voltage measuring apparatus, the common mode error occurs in which the voltage for measurement that should be held in the capacitor 103 is offset by the leakage current Ia caused by the disturbance voltage En, and thereby to degrade the accuracy of voltage measurement.
When the measuring method of flying capacitor method of the second conventional apparatus is employed, the voltage measuring apparatus of first conventional can be simplified in construction thereof. However, even when the measuring method of the second conventional apparatus is employed, expensive isolated driving type analog switch elements totaling 96 in number must be used for the 24 modules (stacked type voltage source), and further improvements have been needed in terms of the cost, size, and reliability.
Next, a third conventional voltage measuring apparatus will be described with reference to FIG. 24.
FIG. 24 is a circuit diagram showing a configuration of a third conventional voltage measuring apparatus.
As shown in FIG. 24, in this third conventional voltage measuring apparatus, the buffer circuit 105 shown in FIG. 23 is replaced by a differential amplifier 106, and a resistor 107 is provided for settling the potential of the capacitor 103 within the input operating range of the differential amplifier 106. The resistor 107 consists of series connected resistors 107a and 107b, and an intermediate terminal therebetween is grounded.
In the third conventional voltage measuring apparatus, the leakage currents Ia and Ib occur as in the second conventional apparatus shown in FIG. 23. However, in the third conventional voltage measuring apparatus, the resistors 107a and 107b are chosen to have the same value, so that the leakage currents Ia and Ib flow into ground potential through the respective resistors 107a and 107b without passing through the capacitor 103. Accordingly, in the third conventional voltage measuring apparatus, the common mode error does not occur.
However, the third conventional voltage measuring apparatus has conflicting design constraints such that, from the viewpoint of the settling time required in the differential amplifier 106 and determined by the resistance value of the resistor 107 and the capacitance of the first switching device 102 in the OFF-state, the resistance value of the resistor 107 should be made small in order to shorten the time required for the settling. On the other hand, of the viewpoint of the voltage leakage from the capacitor 103 in a period from the time when the second switching device 104 turns on to the voltage measurement is completed, the above-mentioned resistance value should be made large. The reason why is to reduce the voltage drop by the resistor 107. As a result, in the third conventional voltage measuring apparatus, the selection and determination of the resistance value has not been able to be made easily, either leading to an increase in the settling time or causing a voltage leakage error due to the voltage leakage, and it has thus been difficult to improve the performance of the measuring apparatus.
Next, a conventional voltage measuring apparatus for measuring the voltages of a plurality of voltage sources, each in an electrically isolated manner, will be described with reference to FIG. 25. This fourth conventional voltage measuring apparatus is described in Japanese Unexamined Patent Publication No. 9-1617.
FIG. 25 is a circuit diagram showing a configuration of a fourth conventional voltage measuring apparatus.
In this fourth conventional voltage measuring apparatus shown in FIG. 25, a flying capacitor circuit comprising a capacitor and first and second switches is connected to each one of a plurality of voltage sources 111, 112, 113, etc. as well as the one shown in FIG. 23. To the voltage source 111, for example, are connected the first switching device 121, the capacitor 131, and the second switching device 141 in this order. Further, in the fourth conventional voltage measuring apparatus, both ends of each of the second switches 141, 142, 143, etc. are connected to an A/D converter 150 which in turn is connected to a digital counting circuit not shown.
In the fourth conventional voltage measuring apparatus, first, while holding the second switching devices 141, 142, 143, etc. in the OFF-state, the first switching devices 121, 122, 123, etc. turn on, thus charging the capacitors 131, 132, 133, etc. with the voltages of the corresponding voltage source 111, 112, 113, etc. Next, while holding the first switching devices 121, 122, 123, etc. in the OFF-state, the second switching devices 141, 142, 143, etc. sequentially turn on, thereby transferring the terminal voltages of the capacitors 131, 132, 133, etc. to the A/D converter 150. In this way, in the fourth conventional voltage measuring apparatus, the voltage from each of the plurality of voltage sources is measured while maintaining the electrical isolation between each voltage source and the counting circuit by the phase relationship between the operations of the respective switches.
However, since the fourth conventional voltage measuring apparatus requires as many flying capacitor circuits as there are voltage sources, it has not been possible to simplify the configuration of the measuring apparatus. Furthermore, as in the second conventional apparatus, the common mode error occurs, causing a degradation of the voltage acquisition accuracy at each capacitor and hence, a degradation of voltage measurement accuracy.
As described in the above, in the conventional voltage measuring apparatuses, it has not been possible to simplify the configuration of the measuring apparatus; furthermore, it has been difficult to alleviate performance problems such as common mode error, settling time, or voltage leakage error.
An object of the present invention is to provide a voltage measuring apparatus that can easily improve the accuracy of voltage measurement by alleviating performance problems such as common mode error, settling time, and voltage leakage error, and can simplify the configuration of the apparatus.
To achieve the above object, a voltage measuring apparatus according to the present invention comprises: a capacitor configured of a plurality of capacitor elements and divided at a connection point into two sections of the same capacitance; a first switching device for connecting a voltage source to be measured to both terminals of the capacitor; a differential amplifier; a second switching device for connecting both terminals of the capacitor to inputs of the differential amplifier; and a third switch for connecting the connection point in the capacitor to signal reference potential of the differential amplifier in time synchronization with the second switching device.
With the above-mentioned configuration, the accuracy of voltage measurement can be easily improved by alleviating the above-described performance problems.
A voltage measuring apparatus according to another aspect of the invention comprises: a capacitor; a first group of switching devices for selectively connecting both terminals of each of a plurality of voltage sources to be measured to both terminals of the capacitor; and a second switching device connected to both terminals of the capacitor.
With the above-mentioned configuration, the voltage measuring apparatus for making measurements on a plurality of voltage sources can be made simple in configuration, and furthermore, the accuracy of voltage measurement can be easily improved by alleviating the above-described performance problems.
A voltage measuring apparatus according to still another aspect of the invention comprises: (N+1) voltage detection terminals connected to N (N is an integer) series connected voltage sources; a capacitor; a first multiplexer for selectively connecting odd-numbered ones of the voltage detection terminals to one terminal of the capacitor; a second multiplexer for selectively connecting even-numbered ones of the voltage detection terminals to the other terminal of the capacitor; a second switching device connected to both terminals of the capacitor; and a polarity corrector for making voltages from odd-numbered ones of the voltage sources the same in polarity as voltages from even-numbered ones of the voltage sources.
With the above-mentioned configuration, the voltage measuring apparatus for measuring the voltage of each individual one of stacked voltage sources can be made simple in configuration, and furthermore, the accuracy of voltage measurement can be easily improved by alleviating the above-described performance problems.
The novel features of the invention will be hereinafter fully described and particularly pointed out in the appended claims, and the construction and details of the invention, together with other objects and features thereof, will become better understood and appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings.